This sounds cool, though I should point out that some item combinations can get shields dangerously high.
Warden's T3 Willpower (>350) + Magic Carpet (400) + Enchanter's Barrier (300) + Superior Cooldown (150) + Concussive Ward (150) =
1,350 minimum spirit shield health.
I just picked items I usually run in a build: if you added Diviner's Kevlar, Shadow Weave and Veil Walker it goes up to 2,650. It can go higher then that too since Warden's shields scale with spirit and there are more items that grant spirit shield buffs, and this isn't even touching on bullet shields. Getting hit by 2,650 instantanious damage for defending yourself is pretty crazy, even if you've got spirit resistances.
You could balance it out if the explosion's damage is just a percentage of the maximum shield health, and if the item has a cooldown after one explosion so it can't be abused with Veil Walker's recharge.